The invention relates to an NMR measuring configuration with a temperature control device for an NMR sample vial filled with a solid and/or liquid sample substance, which is disposed at a measuring position in an NMR spectrometer in a measurement space surrounded by NMR coils and around which temperature-control fluid flows, which is temperature controlled in the supply flow to the measurement space by a closed-loop-controlled heater, wherein at least one temperature sensor is provided whose temperature-sensitive measurement head is positioned in the spatial vicinity of the NMR sample vial and at least partially projects into the measurement space, while the supply wires to the measuring head of the temperature sensor are disposed in a space that is separate from the measurement space.
Such an NMR measuring configuration is known from U.S. Pat. No. 4,266,194 (=reference [1]).
Nuclear magnetic resonance (NMR) spectroscopy is a powerful method of instrumental analytics. In NMR spectroscopy, radio-frequency (RF) pulses are irradiated into a measurement sample that is disposed in a strong, static magnetic field, and the RF response of the measurement sample is measured. The information is gained integrally over a certain region of the measurement sample, termed the active volume. The measurement sample is measured by the probe head.
The temperature of the sample (Tprobe) influences the results of the NMR measurements. For high-quality measurements, the temperature is typically set using a temperature-control unit and, if possible, kept constant in space and time over the active measurement volume. NMR measurements are typically performed both with heated and with cooled samples. (If the sample is to be cooled to below room temperature, a sufficiently cold temperature-control fluid flow is guided in the supply flow tube and is heated to the target temperature by the heater.) The spatial temperature gradient over the active measurement volume and the stability of the sample temperature over time have a considerable influence on the quality of the NMR measurements.
Temperature-control units for minimization of the temperature gradient in the active measurement volume are known from DE 10 2010 029 080 A1 [2] and DE 40 18 734 C2 [3].
The temperature of the temperature-control fluid is measured using one or more temperature sensors. These sensor temperatures (Tsensor) are processed in a closed-loop control. This closed-loop control controls the heating power of the heater, which is located in the supply flow tube of the temperature-control fluid.
The aim of the closed-loop control is to set the desired target temperature in the NMR sample as well as possible. In prior art ([1], [3]), the temperature sensors are located outside the sample vial. The temperature sensors do not therefore measure the temperature of the sample but the temperature of the gas flowing around it. The difference between the sample temperature and the sensor temperature (ΔTp) is compensated for by suitable calibration (where ΔTp=Tprobe-Tsensor).
The calibration is, however, not equally valid for the entire temperature range of the sample, which is typically −200° C. to +200° C. Minimization of the deviation ΔTp over the whole temperature range is therefore desirable.
Different types of temperature sensors are known. Thermocouples are in widespread use. These essentially comprise two supply wires of different materials (e.g. type K made of nickel-chromium and nickel-aluminum or type T made of copper and constantan), which are connected at a thermojunction. The thermojunction is positioned in a location at which the temperature is to be measured, the temperature measuring point. The wires and the thermojunction are typically surrounded by an electrically insulating filling material having good thermal conduction properties and surrounded by an electrically conductive sheath. The electrically conductive sheath counteracts the penetration of RF fields of the NMR coil into the interior of the temperature sensor and prevents the thermojunction from being directly heated by the influence of the RF fields and the RF currents from being able to advance further along the supply wires as far as the evaluation electronics of the temperature sensor as conducted interference. A further function of the electrically conductive sheath is to prevent the penetration of RF interference originating from, for example, radio and television transmitters and other unspecified sources of interference into the measuring head as well as possible by connecting the sheath, if possible over its entire length, to the ground of the measuring head, with a low-impedance connection to the outer enclosure of the measuring head being of decisive importance. The electrically conductive sheath typically exhibits high thermal conductivity.
The supply wires and the sheath exhibit longitudinal thermal conduction, which depends on the material and geometry. Longitudinal thermal conduction means thermal conduction that is perpendicular to the conductor cross-section. Transverse thermal conduction in the radial direction also occurs. The temperature sensor projects into the measurement space with an immersion depth ET and the temperature-control fluid surrounds or flows around it. Because of the longitudinal and transverse thermal conduction and the finite ET, the temperature sensor measures a mixed temperature comprising the temperature of the fluid flowing around the sensor tip and the temperature prevailing along the supply wires, especially outside the measurement space. The deviation of the mixed temperature from the temperature of the fluid in the absence of the sensor is unwanted and should be kept as small as possible. When the outside temperature changes, this causes the mixed temperature measured by the sensor to also change. This change is included in the control loop of the temperature control and causes a change in the Tin of the fluid flowing into the measurement space and ultimately in the sample temperature Tprobe. The ratio of the changes is termed the temperature penetration factor D of the laboratory temperature to the sample temperature:D=ΔTprobe/(ΔTprobe+ΔTlab)
Where:
ΔTprobe=temperature change in the measurement sample
ΔTlab=change in the laboratory temperature.
One disadvantage of prior art, however, is that typical values for the temperature penetration factor are D= 1/10 . . . 1/20.
The temperature penetration factor D has a direct influence on the quality of the NMR measurements because on a change in the laboratory temperature ΔTlab changes the sample temperature by the factor D·ΔTlab (ΔTlab>>ΔTprobe was assumed). For that reason, an attempt is made to keep D as small as possible.
One way of minimizing the temperature penetration factor is to route the temperature sensor including its supply wires in the supply flow tube (cf. prior art in [1], [6]). However, because the supply flow tube is typically very well thermally insulated toward the exterior, e.g. by means of a glass vacuum vessel ([1]), it has large dimensions and the temperature sensor is therefore far away from the sample. This, in turn, results in a large difference between sample temperature (Tprobe) and sensor temperature (Tsensor).
Another way of minimizing D is to mount the temperature sensors in the measurement space. This possibility is applied in [1]. Because these temperature sensors are typically not completely non-magnetic, they must be kept at a certain distance from the sample vial to avoid magnetic interference. However, the greater the distance, the greater the difference between the sensor temperature and the sample temperature. Moreover, the parts of the supply wires not routed in the measurement volume cause a mixed temperature that deviates from the temperature of the temperature measuring point and therefore increases the temperature penetration factor D.
A further way of minimizing D is to minimize the longitudinal thermal conduction of the sensor from the connection to the thermojunction and to maximize it in the region of the thermojunction as far as the medium, while shielding against irradiated interference, as shown in [5]. The drawback of this method is the relatively complex structure of the sensor. Moreover, minimization of the longitudinal thermal conduction also entails minimization of the thermally relevant cross-sections and use of materials with poorer thermal conductivity, wherein not only RF shielding but also the filler material and the two supply wires must be considered. Depending on further requirements such as technical temperature range, tolerance, and resistance to aging of the sensor, it is not always possible to find an optimum solution because the requirements are sometimes mutually incompatible, in which case it is necessary to assess the advantages and disadvantages of different variants.
The object of this invention is to improve an NMR measuring configuration of the type mentioned in the introduction by the simplest possible technical means such that the temperature penetration factor is minimal and the difference between the sensor and sample temperature (ΔTp) is also minimized.